Key generation method for communication session encryption and authentication system

ABSTRACT

An interactive mutual authentication protocol, which does not allow shared secrets to pass through untrusted communication media, integrates an encryption key management system into the authentication protocol. The server provides ephemeral encryption keys in response to a request during a Session Random Key (SRK) initiation interval. SRK is provided for all sessions initiated in the SRK initiation interval. A set of ephemeral intermediate Data Random Keys (DRK) is associated with each request. A message carrying the SRK is sent to the requestor. A response from the requester includes a shared parameter encrypted using the SRK verifying receipt of the SRK. After verifying receipt of the SRK at the requester, at least one message is sent by the server carrying an encrypted version of one of said set of ephemeral intermediate DRK to be accepted as an encryption key for the session.

REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. patent application Ser. No.10/653,506, entitled COMMUNICATION SESSION ENCRYPTION AND AUTHENTICATIONSYSTEM, invented by Mizrah, and filed on the same day as the presentapplication.

The present application is related to U.S. patent application Ser. No.10/653,500, entitled KEY CONVERSION METHOD FOR COMMUNICATION SESSIONENCRYPTION AND AUTHENTICATION SYSTEM, invented by Mizrah, and filed onthe same day as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to security of authentication and datatransmission over untrusted communication media in client-servercomputer, network, and other architectures, and more particularly toencryption key management systems and authentication protocols.

2. Description of the Related Art

Electronic networks of interconnected devices and users continue to growwith unprecedented rate. They have become foundations for vitallyimportant infrastructures enabling e-commerce, communications, corporateand government operations, healthcare, education, and other importantareas. This phenomenon was actively studied and commercialized duringthe last quarter of the 20^(th) century, and there is every indicationthis activity will intensify well into the 21^(st) century.

There are various parties involved in remote relationships overdistributed electronic networks. Most known representations arebusiness-to-business (b2b), business-to-consumers (b2c), andpeer-to-peer (p2p), describing scaled-down to hardware devicescommunication, for instance, peer router to peer router, ordevice-to-device (d2d). One of the fundamental problems for continuedgrowth of electronic networks and their efficient utilization isestablishing trust between remote counterparts in b2b, b2c, d2d, andother interrelating over network parties. It is common knowledge thatcomputer network intruders (or intruding organizations) causeever-growing direct economic losses to enterprises and individualconsumers. They significantly undermine the progress in applying networktechnologies to certain areas, especially related to parties havinglegal and financial responsibilities, and national security.

Trust to remote humans or devices, interacting over electronic networks,has two components. The first component is identification andverification of the parties at the beginning of the communicationsession (mutual authentication). The second component is associated withtrust to information transferred during the communication session overuntrusted communication media (communication lines). It includes thefollowing specific requirements—confidentiality (none can read themessage except the intended recipient), integrity (none altered,tampered with, or modified the message against the original), andnon-repudiation (the sender cannot deny the fact of having sent themessage).

Authentication and cryptography are key enabling technologies employedto satisfy the security requirements listed above. Authenticationfactors are typically characterized as “what user knows” (for instance,passwords, PINs), “what user has” (for instance, hardware token, smartcard), and “what user is” (particular biometric traits; for instance,fingerprints, voice patterns, etc.). Passwords are the most ubiquitousover electronic networks as an authentication factor due to ease of use,low cost and easy electronic deployment. Most of the strong (two-, orthree-factor) authentication systems are still using passwords or PINsas one of the system authentication factors.

However, passwords provide low security due to insufficient protectionagainst numerous known intruding attacks on databases where thepasswords are residing, social engineering attacks, videotaping or“shoulder surfing” attacks during password entry stages, memory sniffingattacks, Trojan horse attacks, and network sniffing attacks. Perhaps,the latter are the most dangerous attacks as a distributed electronicnetwork (like Internet) has numerous access points. There areauthentication systems transmitting passwords in clear text (forinstance, Password Authentication Protocol (PAP) RFC 1334-2, Telnet, andFTP). Certainly, there is no protection at all in such cases. Moreprotected authentication systems transmit encrypted passwords overelectronic networks.

There are several approaches in transferring an encrypted password. Thefirst one is based on the one-way encryption—calculating the password'shash value with one of the standard hashing algorithms (for example,SHA-1 Secure Hash Algorithm, FIPS PUB 180-1, Secure Hash Standard, 1995,Apr. 17, or MD5 Message Digest Algorithms, RFC 1320 and RFC 1321, April1992, by Ronald L. Rivest) at both client and server locations. Theclient transmits the hashed password (of the user at the clientplatform) to the server, where it is compared with the password of thesame client (the same user at the client platform) from the databaseconnected to the server (typically, user passwords are already stored inpassword files in hashed form for database protection; that is why thereis no need to perform text-to-hash encryption operation). Unfortunately,the progress in integrated circuit (ASIC, FPGA, etc.) design andmanufacturing drastically reduced protection of hashed passwords, asdictionary or brute force computer processing attacks became extremelyefficient. It is worthy to note that sometimes intercepting a hashedpassword is sufficient enough to break the system without learning theactual password.

There are more sophisticated authentication systems based onChallenge-Handshake Authentication Protocol (CHAP, for instance, RFC1334-3, RFC 1934, RFC 2759) used by Microsoft for Windows NT remotelog-in. The server (the authenticator) sends the “challenge” to theclient (the peer), where the message gets encrypted using the client's(the peer's) password. Actually, the “challenge” sent to the clientplatform is then encrypted at the client location three times using thefirst seven bytes of the password's hash value as the first DES key(Data Encryption Standard and other known encryption algorithms used fordata encryption and decryption described in Bruce Schneier, AppliedCryptography, Second Edition, John Wiley and Sons, Inc., at pp. 233-560,(1996)); the next seven bytes of the password's hash value used as thesecond DES key, and the remaining two bytes of the password's hash valueconcatenated with five zero-filled bytes used as a third DES key.Eventually, three 64-bit “responses” (the “challenge” encrypted with DESkeys as described above) are sent back to the server (theauthenticator), where they are compared with the similar outputscalculated at the server. If the values match, the authentication isacknowledged; otherwise the connection should be terminated.

Passwords (client/server shared secrets) in CHAP never entercommunication lines in either form. This is a serious security advantageof this protocol. Also, CHAP prevents playback attacks by using“challenges” of a variable value. The server (the authenticator) is incontrol of the frequency and timing of the “challenge”. CHAP assumesthat passwords are already known to the client and the server, and areeasily accessible during a CHAP session. However, frequent usage of thesame static encryption keys derived from a password on the client host,and applied to encrypt even random “challenge” numbers sent in cleartext to the client, raises some security concerns. It provides ampleopportunities for intruders, sniffing the network with the followingoffline computer data processing attacks.

Various modifications of client/server authentication employing achallenge/response protocol are disclosed in Bellovin et al., U.S. Pat.No. 5,241,599, Kung et al., U.S. Pat. No. 5,434,918, Pinkas, U.S. Pat.No. 5,841,871, Hellman, U.S. Pat. No. 5,872,917, Brown, U.S. Pat. No.6,058,480, Hoffstein at al., U.S. Pat. No. 6,076,163, Guthrie et al.,U.S. Pat. No. 6,161,185, Jablon, U.S. Pat. No. 6,226,383, Swift et al.,U.S. Pat. No. 6,377,691, Brown, U.S. Pat. No. 6,487,667, Jerdonek, U.S.Pub. No. 2002/0095507. Some of these patents go beyond security of justonly an authentication process. They explore the opportunity ofutilizing challenge/response type protocols as a basis for an encryptionkey management system. This can extend security for the entirecommunication session duration, allowing for encrypted data transmissionbetween parties once their mutual authentication is completed.

U.S. Pat. No. 5,434,918 and U.S. Pat. No. 6,377,691 appliedclient/server authentication based on different modifications of achallenge/response protocol to exchange secret keys (symmetriccryptography) between parties. There were attempts combiningchallenge/response protocols with well-known encryption key managementsystems. For instance, U.S. Pat. No. 6,076,163 and U.S. Pub. No.2002/0095507 disclose versions of a challenge/response protocolutilizing an authentication and encryption key management system basedon PKI (Public Key Infrastructure (Hellman et al., U.S. Pat. No.4,200,770, and Diffie at al., IEEE Transactions on Information Theory,vol. IT-22, No. 6, Nov. 1976)), whereas U.S. Pat. No. 5,841,871discloses a version of a challenge/response protocol integrated withKerberos (NET, 1988; RFC 1510))—the authentication and encryption keymanagement system.

Another approach would be encrypting passwords (either text or hash)with a secret key (symmetric cryptography) on the client side, beforetransmission, and then, decrypt it on the server side for comparisonwith the password stored in the server-connected database. Though it canbe a viable solution, there are several security requirements makingthis approach a very difficult one to implement. The first issue is howto manage the session secret key distribution between the client and theserver. Otherwise, if the secret keys are statically preset at theclient and the server hosts, they become a security concern bythemselves. Moreover, having static keys for numerous communicationsessions makes encrypted passwords vulnerable against offline computerdata processing attacks. There are protocols, not based on achallenge/response type mechanism, where authentication credentials aredistributed over communication lines with help of PKI. They weredisclosed in Kaliski, U.S. Pat. No. 6,085,320, Kausik, U.S. Pat. No.6,170,058, Kaliski, U.S. Pat. No. 6,189,098, Spies, U.S. Pat. No.6,230,269 and Volger, U.S. Pat. No. 6,393,127. Despite recognizedscientific studies and long-time exposure, PKI and Kerberosauthentication and encryption key management systems have notexperienced a wide industry acceptance due to their complexity, cost,and mandatory requirements to trust artificial third parties (see, forinstance, Gartner QA-18-7301, 13 Nov., 2002, by V. Wheatman, Public-KeyInfrastructure Q&A, and DPRO-90693, 20 May, 2003, by Kristen Noakes-Fry,Public Key Infrastructure: Technology Overview; USENIX, 91, Dallas,Tex., “Limitations of the Kerberos Authentication System”, by Steven M.Bellovin and Michael Merritt). SSL (Secure Socket Layer, based on PKIprotocol developed by Netscape Communications in 1994) is also known forits security deficiencies, high cost and complexity in assuring “clientbrowser”/“Web server” encrypted communication (see, for instance,Gartner T-16-0632, 3 Apr., 2002, by J. Pescatore and V. Wheatman, andFT-178896, 15 Aug., 2002, by J. Pescatore). Hence, there is asignificant interest in exploring other encryption key managementsystems, similar to the challenge/response authentication protocolsmentioned above, for instance, Fielder, U.S. Pat. No. 6,105,133, Alegre,U.S. Pat. No. 6,199,113, and Venkatram, U.S. Pat. No. 6,367,010.

Aspects of this invention are particularly concerned with security ofauthentication systems and encrypted information exchange overdistributed computer networks. Prior art encrypted authenticationprotocol implementations based on PKI, SSL, and Kerberos exhibitednumerous security flaws and a prohibitive level of complexity and costfor various applications, businesses and organizations. There is asubstantial need for improved and more efficient encryptedauthentication protocols, addressing less complex infrastructuresrequired, and less costly for practical implementation encryption keymanagement systems. These improved encrypted authentication protocolsshould also include secure mutual authentication built into theprotocols; randomly generated session secret keys; new cryptographicalgorithms allowing for scalable security authentication and dataencryption, and further allowing for variation based on the power ofcomputer and network resources.

SUMMARY OF THE INVENTION

In accordance with the present invention, there are two secrets uniquelyshared by either client/server pair, or authenticator/peer pair, andrequired for their mutual authentication. In the preferred embodiments,both secrets will suffice for a “what user knows” type authenticationfactor and either could be in a form of passwords or PINs, though othertypes of shared secrets can be used. Like other challenge/response typeauthentication protocols, where shared secrets are never in transit overcommunication lines, the protocol of the present invention does notallow shared secrets to pass through untrusted communication media. Inorder to avoid transmission of the shared secrets, a new encryption keymanagement system has been integrated into the authentication protocol,becoming an essential part of the protocol itself.

The main function of this encryption key management system is a securedistribution within either client/server pair, or authenticator/peerpair of a secret session random key (the same secret key is used insymmetric cryptography to encrypt and then decrypt digital information).Successful exchange of this encryption key enables secure resolution oftwo fundamental tasks. First, it allows for secure transit of theprotocol data over communication lines in encrypted form, permittingexplicit mutual authentication of the connected parties. Second, thepost-authentication stage of the communication session can use secureencryption for the data exchange, since each party has already obtainedthe secret session random key.

A series of new algorithms has been developed in the present inventionand built into the new encryption key management system mentioned above.There is an algorithm (Time Interplay Limited Session Random Key (SRK)Algorithm (TILSA)) for generating and eventually obliterating arrays ofsession secret random keys. It starts long before the session begins andkeeps processing these arrays during each communication session and wellbeyond it. At the same time, this algorithm allows concurrentcommunication between a number of client/server or authenticator/peerpairs with the same keys in the generated arrays (the multi-threadingtechnology).

Another algorithm (Key Encryption/Decryption Iterative Algorithm(KEDIA)) is initialized, provided there is a request for connection. Itinitiates an iterative sequence of messages from the server to theclient and back to the server, each containing a consecutive sessionsecret random key, encrypted with the session secret random keypreceding the encrypted one in the array, and sent to the client in theprevious message. The client can decrypt any following message andobtain an intermediate session secret random key from the array,provided the client could decrypt the previous message. The iterationscontinue until client/server (or authenticator/peer) mutualauthentication is completed, and the Final Secret Key (FSK) is exchangedbetween the parties. More particularly, client/server (orauthenticator/peer) authentication credentials and FSK eventual highsecurity are achieved by applying, during each cycle of key encryptionat the server (and its decryption at the client platform), either ofByte-Veil-Unveil (ByteVU), Bit-Veil-Unveil (BitVU), orByte-Bit-Veil-Unveil (BBVU) algorithms. Each of these algorithmsdisassembles message bytes, or bits, or both bytes and bits incombination, together at the server and reassemble them at the clientaccording to a certain procedure, which is started with the pair'sshared secret. In other words, the client/server or authenticator/peerpair employs their shared secret to first build the session “securitybridge” over the untrusted communication medium until “the bridge” isbelieved secure enough. Then, the authentication credentials can besafely tested with ByteVU, BitVU, or BBVU algorithms at the respectivecounterparts for the final mutual authentication, enabling thecommunication session. Otherwise, if the mutual authentication is notcompleted, the communication session is terminated.

In one aspect of the invention, the client/server authenticationprotocol (Message Encrypt/Decrypt Iterative Authentication (MEDIA)protocol, which includes the encryption key management system describedabove), is highly resilient against session eavesdropping attacks,replay attacks, man-in-the-middle attacks, online and offlinecomputer-processing attacks (like a dictionary attack or a brute forceattack), and session hijacking attacks. Inability to successfullycomplete the MEDIA protocol can be regarded as intrusion detection (ifthere are more than just a few failed attempts from the same clientcaused by mistyping the entry data by a user on the client platform, orinaccurately set up hardware authentication credentials).

In another aspect of the invention, the MEDIA protocol ends up with aFSK secret key, which can be used beyond the client/serverauthentication protocol stage of the communication session forencrypting data in transit and decrypting it upon arrival either to theserver, or to the client. Security of FSK, and authenticationcredentials (client/server shared secrets) are guarded by five securitytiers of the MEDIA authentication protocol and can be scaled with theclient and server platforms' CPU power and the network throughput. Inorder to enhance security, FSK, as well as the entire series ofpreceding FSK session iterative random secret keys of the MEDIAprotocol, are never transmitted over untrusted electronic communicationmedia in their original form, or as their hash equivalents.

In yet another aspect of the invention, the MEDIA protocol contains theencryption key management system, integrated into the protocol,represented by TILSA, KEDIA and ByteVU, BitVU, or BBVU algorithms. Theircollective utilization assures randomly generated arrays of sessionsecret keys, having limited life time and enabling efficient keyencrypt/decrypt iterative messaging procedures, employing for eachinstance of iteration (each message encrypted on the server anddecrypted on the client) a shared secret (password, PIN, or pattern)known only to the client and to the server. Moreover, the shared secrets(for instance, client and server passwords) are never transmitted overuntrusted communication media in any form.

In still another aspect of the invention, the five security tiers of theMEDIA protocol provide for a message confidentiality (no one can readmessages; this is increasingly true with the increased number of SRK inthe TILSA and the number of message iterations in the KEDIA). Messageintegrity is preserved because, if an intruder altered or in some waytampered with the message in the conversion array, potentially availableto an intruder while it is in transit on communication lines, then itwill be impossible for ByteVU, BitVU, or BBVU algorithms to reassemblethe encrypted keys or authentication credentials, either at the clientor at the server. Message non-repudiation is guaranteed by the mutualauthentication mechanism (the fifth security tier)—without exceptiononly the client and the server know their shared secrets andrespectively could send a message.

In a further aspect of the invention, the post-authentication part ofthe MEDIA session continued with FSK can also employ such messageintegrity control technique as encrypting with FSK the message hash,before the message is encrypted with FSK. Then the message hash can bedecrypted with FSK on the receiving end, and compared with the samemessage, hashing it after having the message decrypted with FSK.

BRIEF DESCRIPTION OF THE DRAWINGS

Additional objects, features, capabilities and advantages of the presentinvention will be apparent from the following detailed description whenread in conjunction with the accompanying drawings in which:

FIG. 1 is a graphic illustration of the Time Interplay Limited SRK(Session Random Key) Algorithm (TILSA) according to the presentinvention.

FIG. 2 is a graphic illustration of the Array of Data Encryption Keys(ADEK) branch of the TILSA algorithm according to the present invention.

FIG. 3 is a graphic illustration of the Key Encryption/DecryptionIterative Algorithm (KEDIA) according to the present invention.

FIG. 4 is a graphic illustration of the KEDIA typical message encryptionat the server and its decryption at the client applying one ofByte-Veil-Unveil (ByteVU), Bit-Veil-Unveil (BitVU), orByte-Bit-Veil-Unveil (BBVU) algorithms according to the presentinvention.

FIG. 5 is a block diagram of the Byte-Veil-Unveil (ByteVU) algorithmaccording to the present invention.

FIG. 6 is a block diagram of the Bit-Veil-Unveil (BitVU) algorithmaccording to the present invention.

FIG. 7 is a block diagram of the Byte-Bit-Veil-Unveil (BBVU) algorithmaccording to the present invention.

FIG. 8A is the Message Encrypt/Decrypt Iterative Authentication (MEDIA)protocol (the server side) according to the present invention.

FIG. 8B is the Message Encrypt/Decrypt Iterative Authentication (MEDIA)protocol (the client side) according to the present invention.

FIG. 9 illustrates the Graphical User Interface (GUI) enablingclient/server mutual authentication at the client platform according tothe MEDIA protocol, and a graphical illustration of the distributedprotected network resources, including the authentication server, andthe user base for which the MEDIA protocol is used, according to thepresent invention.

DETAILED DESCRIPTION

According to the present invention, there are shared secrets (severalsecrets are needed in strong authentication cases and also in a case ofmutual authentication) between two parties attempting to establish trustover untrusted electronic communication media. Shared secrets areusually established during an account open procedure. Though the serverpassword could be shared by the plurality of users, it is assumed,without sacrificing any generality of the disclosed authenticationprotocol, that the preferred embodiment of this invention is to providea unique server password for each user. Account set/reset onlineautomated utilities would greatly facilitate establishing uniquelypersonalized server and user passwords. Client/server or d2d(authenticator/peer) communication sessions would be typical cases,though the client/server protocol would remain the preferred embodiment.There are no limitations on the nature of the shared secrets used. Theycould be “what user knows” secrets, for example, passwords, or “whatuser has” secrets, i.e., tokens and smart cards, or, alternatively,“what user is” secrets, for example, biometrics. However, the preferredembodiments would relate to secrets in the category of “what userknows”. Also, there are no limitations on the network layer over whichthe authentication protocol is established—it could be TCP/IP stack,IPsec, or other communication protocols. Nevertheless, the preferredembodiments will assume HTTP (RFC 2068 Hypertext TransferProtocol—HTTP/1.1 January 1997). Also, the invention impliescontemporary object-oriented software technologies like Java, C++, andNET, providing multi-threading, serialization, servlet and applettechniques, library of cryptographic algorithms, GUI (Graphical UserInterface) capabilities, and connectors/drivers like JDBC to standardcommercial databases.

FIG. 1 is a graphic illustration of the Time Interplay Limited SRK(Session Random Key) Algorithm (TILSA) according to the presentinvention. Before any communication session starts, the server-placedlogic continuously and periodically generates (Session Random KeyGenerator 1005) an array (Array of Session Keys (ASK) 1013) of SessionRandom Keys (SRK) 1011—secret keys (symmetric cryptography). Each keyhas two different lifetimes. The first lifetime (LT1) is the lifetimefor establishing a client/server communication session, provided thereis a request from a client or plurality of clients (Client 1 1003,Client 2 1007, . . . , Client N-1 1008, and Client N 1009) during LT1 toinitiate a communication session. Each client can establish acommunication link 1015, 1006, . . . , 1016, 1017 to Web Server 1002 andCompute/Applications Server 1001 through communication network 1004. Thebeginning of LT1 1014 is synchronized with each SRK 1011 generation,placing it into ASK 1013.

For instance, in FIG. 1, SRK1 appears in ASK 1013 at the time mark “0minute”, and at the moment that time mark 1 minute LT1 of SRK1 hasexpired, though SRK1 remains inside ASK 1013. SRK Generator 1005 at thismoment generates SRK2 and places it into ASK 1013. By the time mark 2minutes SRK2 LT1 has expired, even though SRK2 remains inside ASK 1013.Again, at this time SRK Generator 1005 generates and places into ASK1013 SRK3, which LT1 becomes expired at the 3 minutes mark. Thisprocedure is periodically repeated as long as SRK Generator 1005 is on.Client 1 1003 and Client N 1009 made a connection request during thetime interval between time mark 4 minutes and time mark 5 minutes, sinceSRK Generator 1005 began generating SRK 1011 and filling them into ASK1013. The only SRK 1011 not yet expired LT1 in ASK 1013 during this timeinterval is SRK5. Therefore, SRK5 is used to establish communicationsessions with these clients. Similarly, Client 2 requested acommunication session between time mark 8 minutes and time mark 9minutes, whereas Client N-1 requested a communication session betweentime mark 1 minute and 2 minutes. Hence, the SRK 1011 used to establishthese communication sessions are, respectively, SRK9 and SRK2.

Once LT1 is expired, the server generates and places into ASK 1013another SRK 1012, which LT1 is just started. SRK1011 second life timeLT2 defines the life time inside the limited size ASK 1013. The maximumsize of ASK 1013 can be characterized with the parameter Nmax whichindicates maximum number of SRK 1011 in ASK 1013 possible (for instance,Nmax=5 in FIG. 1). Typically, LT1<LT2, and in the most preferredembodiment LT1 can be derived as LT1=LT2/Nmax. Without sacrificing anygenerality limitations of TILSA, LT2 was chosen, for example, to beequal to 5 minutes in FIG. 1. Then, LT1 according to the formulaepresented above, is equal to 1 minute. After LT1 expired, for any givenSRK 1011, the key has LT2−LT1 time remaining to support communicationsession threads having been initiated during LT1. Once LT2 expired, SRK1011 is removed from ASK 1013, effectively canceling any furtherparticipation of this particular SRK 1011 in the parties' communicationsession engagements. Certainly, each SRK 1011 can be used to originatemultiple threads of communication sessions with each Session ElapsedTime (SET) less or equal to LT2−LT1. However, SET=LT2−LT1 is thepreferred embodiment. Without sacrificing any generality limitations ofTILSA, SET=4 minutes in FIG. 1. Taking SRK5 in FIG. 1 as an example ofany SRK 1011 genesis, one can note that SRK5 is the last key to fulfillASK 1013 to its maximum size Nmax=5, and SRK5 appears inside ASK 1013 atthe 4-minute mark, since SRK Generator 1005 began generating SRK 1011and filling them into ASK 1013. Then, during SRK5 LT1=1 minute, the keycan be engaged into initiating multiple communication session threadswith the clients requesting connections. From time mark 5 minutes, anduntil time mark 9 minutes, SRK5, in accordance with SET=4 minutes, iskept inside ASK 1013 available to support communication session threadsstarted during SRK5 LT1. During this particular time interval, from timemark 5 minutes to time mark 9 minutes, SRK1, SRK2, SRK3, and SRK4 in ASK1013 are being gradually replaced every minute by SRK6, SRK7, SRK8, andSRK9, respectively. Eventually, at time mark 9 minutes SRK5 is canceled,ultimately being replaced by SRK10.

This Time Interplay Limited SRK Algorithm (TILSA) is the first securitytier of the authentication protocol, assuring supply of SRK 1011 toinitiate any client/server communication session. However, the time toinitiate a session (approximately one minute, without sacrificing anygenerality limitations of TILSA) and the time to continue the sessionauthentication protocol (possibly several minutes, without sacrificingany generality limitation of TILSA) are quite limited for any given SRK1011, thus hindering a possible intruding activity.

FIG. 2 is a graphic illustration of the Array of Data Encryption Keys(ADEK) branch of the TILSA algorithm according to the present invention.The essential part of TILSA is generating (Data Random Key Generator2005) an array of Data Random Keys (DRK) 2013—secret keys to support theauthentication session for any particular SRK 1011 starting acommunication session thread. This array of DRK (Array of DataEncryption Keys (ADEK) 2012) is regenerated and specifically attributedto each SRK 1011, together and concurrently with originating any new SRK1011 with the logic located on the server side; explaining why there isno latency in the DRK supply during a client/server encryptedauthentication session. The number of DRK 2013 in ADEK 2012 is fixed,acting as a security parameter for the MEDIA authentication protocolbeing presented. Each ADEK 2012 can be used for a plurality of threadsinitiated with a particular SRK, to which this ADEK 2012 belongs. TheADEK 2012 lifetime is limited and equal to the lifetime of theoriginated SRK 1011 in ASK 1013, being LT2. Deleting SRK 1011 from ASK1013 inevitably deletes ADEK 2013, corresponding to this SRK 1011.

Once the client requested a connection to the server supported by theuser name of the user on the client platform (or the client host name),a suitable SRK 1011, accompanied by LT1, not yet expired, is sent to theclient by the server. In the most preferred embodiment of thisinvention, SRK 1011 is sent to the client in a compiled form (forexample, as a class file). This is the second security tier of theauthentication protocol, in view of the fact that reengineering acompiled key given a short SRK 1011 lifetime LT2 is a formidable task.Therefore, the first two security tiers make SRK 1011 quite resilient tothe on line attacks during the session time, because of incommensuratetimes to reengineer SRK 1011 versus SRK 1011 expiration time LT2.However, SRK 1011 is still vulnerable against off line attacks and needsto be enhanced further to avoid any loss of authentication credentialsand the eventual session Final Secret Key (FSK).

Since SRK 1011 is sent to the client as the first message, the logiclocated on the server and on the client sides generates a series ofmessages having been sent from the server to the client, and back to theserver with the following Key Encryption/Decryption Iterative Algorithm(KEDIA). FIG. 3 is a graphic illustration of the KeyIncryption/Decryption Iterative Algorithm (KEDIA) according to thepresent invention. In step 1 3005, client 3002 sends a connectionrequest to server 3001 over communication network 3003. In step 2 3006,SRKi (with the currently active LT1 i—between time mark i−1 minutes andtime mark i minutes) is sent to client 3002, and stored there,initiating the communication interface. In step 3 3007, client 3002enters the user name, the user password, and the server password, if itis a user at the client platform 3002, or the host name, the host ID,and the server password, if it is the client platform (the peer). Instep 4 3008, the user name (or the host name) is hashed, encrypted withSRKi and sent to server 3001, while the user password (or the host ID)and the server password were not sent, remaining at client 3002.

In step 5 3009, server 3001 checks the validity of the user name (or thehost name), obtained in the step 4, through the database to which it isconnected. The session is terminated, if the user name (or the hostname) is not valid. Otherwise, server 3001 in step 3009 sends DRK1encrypted with SRKi to client 3002, where DRK1 is decrypted with SRKi,and stored at client 3002. During the same step 3009, client 3002 sendsa DRK1, which is converted to its hash equivalent and encrypted withDRK1, to server 3001. This message confirms to server 3001 that client3002 obtained and decrypted DRK1, and it is ready for receiving anothersecret key. In step 6 3010, server 3010 first decrypts hashed DRK1,received in step 5 from client 3002, with DRK1. If DRK1 is correct,server 3001 sends DRK2 encrypted with DRK1 to client 3002, where DRK2 isdecrypted with DRK1, and stored at client 3002. Otherwise, thecommunication session is terminated. During the same step 6 3010, client3002 sends a DRK2, converted to its hash equivalent, and encrypted withDRK2, to server 3001. This message confirms to server 3001 that client3002 obtained and decrypted DRK2, and it is ready for receiving anothersecret key.

This iterative process continues up to step n 3014. Parameter n isactually the maximum number of DRK 2013 in ADEK 2012 (FIG. 2), andshould be chosen for any practical implementation of this encryptedauthentication protocol. Then, assuming DRKn−1 hash received from client3002 in the previous step n−1 is correct, server 3001, sends DRKn,encrypted with client 3002 hashed password (taken from server database3004, as server 3001 knows from step 4 3008, the identification of theclient (or the user on the client platform)) to client 3002, where DRKnis decrypted with the client 3002 password, stored at the client side instep 3. During the same step n, client 3002 sends to server 3001 hashedDRKn encrypted with the client 3002 password, stored at client 3002 atstep 3 and converted to its hash equivalent. Step n is an importantfirst phase towards client/server mutual authentication. Indeed, theclient can decrypt DRKn only in the case where client 3002 knows theuser 3002 password. Then, client 3002 encrypts hashed DRKn with theclient 3002 hashed password, as a secret key and sends it back to server3001 in same step n 3014. Having received DRKn encrypted with client3001 password, server 3001 decrypts it with the client 3001 password,and, if it is correct, server 3001, in step n+1 3015, sends to client3002 DRKn encrypted with hashed server 3001 password as a key.

Certainly, client 3002, already aware of DRKn from the previous step n3014, compares the result of decrypting the last message with the server3001 password, stored at client 3001 in step 3 3007, and converted toits hash equivalent, with DRKn. If they are the same, the client isassured that the communication session is with the correct server, asonly client 3002 and server 3001 know the server 3001 password.Otherwise, the client 3002 terminates the communication session, andintrusion detection is reported. Eventually, during same step n+1 3015,client 3002 sends to server 3001 hashed DRKn encrypted with the serverpassword, stored at client 3002, at step 3 3007, and converted to itshash equivalent. This message, transmitted back to server 3001, meansthat client 3002 has established trust to server 3001. In step n+2 3016,server 3001 decrypts hashed DRKn with the server password from the 3004database connected to the server, and compares the result with DRKn atserver 3001. Depending on the comparison results, server 3001, duringsame step n+2 3016, sends to client 3002 the authentication signal“go/no” encrypted with DRKn−1, stored at client 3002, at the step, priorto step 3014. This completes the client/server mutual authentication andFinal Secret Key (FSK) exchange according to the KEDIA algorithm.

One encryption/decryption algorithm used in an embodiment of theinvention is the Triple Data Encryption Standard block cipher algorithm.Triple-DES (3DES), based upon the Triple Data Encryption Algorithm(TDEA), is described in FIPS 46-3. Other block cipher algorithms arealso suitable, including RC6, Blowfish/Twofish, Rijndael, and AES. See,Bruce Schneier, Applied Cryptography, Second Edition, John Wiley andSons, Inc., cited above.

In this form the KEDIA algorithm, described above as part of theauthentication communication protocol, is the third security tier,efficient against online and offline intruding attacks. Among otherfactors, the security against online attacks is increased due toeffectively extending the time, needed by an intruder to decrypt theentire series of DRK 2013 in ADEK 2012, whereas the ADEK 2012 life timeis quite limited and is actually equal to LT2, the same as for SRK 1011,which originated this ADEK 2013. As mentioned above, the number of DRK2013 in ADEK 2012 is the authentication protocol security parameter andcan be chosen according to the security requirements, considering theactual system CPU and network resources. Security against offlineattacks is assured through the mutual client/server authenticationutilizing shared secrets known only to the client, and to the server.Moreover, the client supposed to perform a strong (two factors)authentication, as the KEDIA algorithm requires the client to entercorrectly the client (the user on the client platform) password and theserver password, unique to the client (the user on the client platform).Important security feature of the KEDIA algorithm are (1) thatclient/server passwords never enter communication lines in either form,(2) client/server pair performs mutual authentication in steps n 3014,n+1 3015, and n+2 3016, and (3) client/server pair exchanges FSKenabling transmitted data encryption during the post-authenticationstage of the communication session.

In the case where an intruder intercepts the last message in step n+2,and somehow knows the format of the “go/no” authentication signals, abrute force computer processing attack could be applied to uncoverDRKn−1. However, the intruder would only gain limited access as DRKn−1is detached from client/server authentication credentials, and from DRKn(which is FSK in this particular embodiment of the KEDIA algorithm).

Therefore, an offline attack is senseless, as the intruder goingbackward through steps 3013, 3010, 3009, and 3008 could find DRKn−2DRKn−3, . . . , DRK1, and eventually SRKi, which are all only one-timesession random keys, and they cannot be reused. Certainly, the intrudercould further decrypt the user name; however, this is not regarded as asecret. The time DRKn−1, operating during the client/servercommunication session, is excruciatingly small for attempting an onlinecomputer processing attack. Even assuming this attack successful, all,the intruder could do is to send to client 3002 an incorrectauthentication signal, which will be visualized in the user's sessionGUI, but would never take effect in the actual system. This is becausethe authentication signal “go/no” enables functionality through theserver 3001 side logic.

The KEDIA algorithm security has been further significantly enhanced byintegrating and synthesizing it with the Byte-Veil-Unveil (ByteVU)algorithm, the Bit-Veil-Unveil (BitVU) algorithm, and theByte-Bit-Veil-Unveil (BBVU) algorithm. All three algorithms are builtaround the idea that every encrypted message in the client/serverdialogue in the KEDIA algorithm is a fixed byte size, relatively small(typically 16 bytes) message. The algorithms employ the fact that theserver already has identified who the client pretends to be, afterreceiving the user name (or the host name) during the initial connectionrequest. At this time, the server finds the password, or another sharedsecret, corresponding to the user name (or the host name), in the serverdatabase 3004, connected to the server. Then, the server employs thisinformation to disassemble only message bytes, or only bits, or thecombination thereof, inside a certain conversion array, making theirreassembling a highly improbable task, unless the client knows theshared secret. In this case, the message, which is the encrypted key, iseasily recovered and eventually decrypted with the secret key, learnedfrom the previous message.

FIG. 4 is a graphic illustration of the KEDIA algorithm. This is atypical message encryption at the server and its decryption at theclient, applying along with encryption and decryption procedures one ofByte-Veil-Unveil (ByteVU), Bit-Veil-Unveil (BitVU), orByte-Bit-Veil-Unveil (BBVU) algorithms, according to the presentinvention. Step 6 3010 has been chosen as a typical message example inthe KEDIA algorithm. According to FIG. 3, during this step, server 3001sends DRK2 encrypted with DRK1 to client 3002, where DRK2 is decryptedwith DRK1, received by client 3002 in the previous step 3009 from server3001. In FIG. 4, step 3010 is split for clarity into two parts 4001 and4002, which are related to preparing the message at server 3001, andtreating the received message at client 3002, respectively. Blocks 4003,4005, 4007, and 4009 depict the process the message is going through,before it is sent to client 3002. DRK2 (for instance, 16 bytes long,secret key to be used with a block-cipher encryption algorithm) issupplied by Server DRK Generator 2005 (see FIG. 2) 4003. In thefollowing step 4005, server 3001, already having identified who claimsto be the user on the client platform, (or what is the claimed clientplatform host name), extracts the respective user password (or theclient host ID) from the database 3004 attached to server 3001.Eventually, according to block 4007, server 3001 uses this informationto trigger operation of one of ByteVU, BitVU, or BBVU algorithms, havingbeen chosen by a particular security system, considering securityrequirements vs. cost trade-offs (time of operations, CPU power ofclient/server platforms, and the network throughput). As a final result4009, the conversion array, containing disassembled DRK2 bytes, or bits,or the combination thereof, gets encrypted with DRK1, and sent to client3002.

Part 4002 of step 3010, related to the received message treatment atclient 3002, is expanded by the series of blocks 4004, 4006, 4008, and4010 in FIG. 4. According to block 4004, client 3002 decrypts theconversion array with DRK1, stored by client 3002 from the previousmessage 3011 from server 3001. Then, client 3002 supplies the userpassword (or the client host ID) which was entered into the KEDIAalgorithm at step 3 3007 (see FIG. 3), enabling reassembling of DRK2from the decrypted conversion array 4006. As it is shown in block 4008,the operation is triggered for one of ByteVU, BitVU, or the BBVUalgorithms, having been chosen on the client side the same one, as onthe server side. Eventually, according to block 4008, either the messagebytes, or bits, or the combination thereof, get reassembled, andfinally, as it is shown in block 4010, DRK2 is reconstructed to itsoriginal form.

In compliance with FIG. 4, each message of the KEDIA algorithm employsadditional treatment as compared to the standard encryption/decryptionoperations. This treatment is triggered by the client/server sharedsecret at the sending and receiving communication channel ends. FIG. 5is a block diagram of the Byte-Veil-Unveil (ByteVU) algorithm accordingto the present invention. Block 5001 shows DRKj, where each byte isseparated from a neighboring byte with a vertical bar. Withoutsacrificing any generality of the ByteVU algorithm, DRKj is assumed tobe a 16-bytes key in FIG. 5. The user password (or the client host ID),supplied by server 3001 in a hashed form, plays a seed role for ServerSequential Random Number Generator (SRNG) 5002. SRNG 5002 generates arandom sequence of integers, and it is the same sequence of integers,each from 1 to 10, for any given seed. In other words, the password (orthe client host ID) and the SRNG sequence of integers are uniquelyassociated. Block 5005 introduces a conversion array which, withoutsacrificing any generality limitations of ByteVU algorithm, has 16 equalsections 5006, 5007, 5008, 5009, and 5010, with 10 bytes per eachsection. FIG. 5 presents an exemplary case, when SRNG 5002 generated 16sequential integers 4, 9, . . . , 2, and 7.

The first integer 4 is used by the logic located by the server 3001 toreplace byte r1,4 in the first section 5006 of conversion array 5005 bythe first byte xh1 of DRKj in 5001. Similarly, the second integer 9 isused by that same logic to replace byte r2,9 in the second section ofconversion array 5005 by the second byte xh2 5012 of DRKj in 5001. Thesame procedure is applied to all integers in the sequence generated bySRNG 5002, until DRKj 15^(th) byte xh15 in 5001 is replacing the 2^(nd)byte r15,2 in the 15^(th) section 5009 of conversion array 5005, andeventually DRKj 16^(th) byte xh16 in 5001 is replacing the 7^(th) byter16,7 in the 16^(th) section of conversion array 5005. Once all bytes ofDRKj are veiled in this manner inside conversion array 5005, the entireconversion array 5005 is encrypted with DRKj−1, and the message is sentto client 3002. At client 3002, the encrypted conversion array isdecrypted with DRKj−1, saved at client 3002, from the previous servermessage (step 3011 in KEDIA, FIG. 3).

The next procedure, reversed as compared to the procedure describedabove on the server 3001 side, is applied. The user password (or theclient host ID) saved at the client platform in step 3007 of the KEDIAalgorithm (see FIG. 3) is supplied in a hashed form as a seed to ClientSequential Random Number Generator (SRNG) 5003, identical to the one onthe server 3001 side. This password (or host ID) triggers SRNG 5003 togenerate the same sequence of integers, as on server 3001 side before 4,9, . . . , 2, 7. Then, the logic placed on client 3002 used the firstinteger 4 to extract DRKj first byte xh1 from the fourth position infirst 10 bytes section 5006 of conversion array 5005, and place it backin the 1^(st) position of DRKj 5001. Consequently, the second integer 9is used to extract DRKj second byte from the 9^(th) position in 10 bytessection 5007 of conversion array 5005, and place it back into the 2^(nd)byte position of DRKj 5001. This procedure is going on, until,eventually, the 15^(th) byte of DRKj xh15 is extracted from the 2^(nd)byte position in 15^(th) 10 bytes section 5009 of conversion array 5005,and placed back into 15^(th) byte position of DRKj 5001 as well as the16^(th) byte of DRKj xh16 5011 extracted from the 7^(th) byte positionin 15^(th) 10 bytes section 5010 of conversion array 5005, and placedback into 15^(th) byte position of DRKj 5001. This completes thereassembling procedure of the ByteVU algorithm to restore DRKj at client3002.

A suitable sequential random number generator SRNG for use inembodiments of the invention is a Java version of the well known “Lehmergenerator.” See, Park & Miller, “Random Number Generators, Good Ones areHard to Find,” Communications of the ACM, Vol. 31, No. 10, (1988), pages1192-1201.

FIG. 6 is a block diagram of the Bit-Veil-Unveil (BitVU) algorithmaccording to the present invention. The BitVU algorithm is a naturalextension of the ByteVU algorithm. Instead of veiling bytes of DRKj, theBitVU algorithm veils bits of DRKj. It is assumed, without sacrificingany generality limitations of the BitVU algorithm, that DRKj bit size is128 bits 6001. Each bit of DRKj in 6001 is separated from a neighboringbit with a vertical bar. Server Sequential Random Number Generator(SRNG) 6002 uses the user password (or the client host ID) supplied bythe server in a hashed form as a seed, allowing for the generation of arandom series of 128 integers with values ranging from 1 to 128 (forinstance, 4, 127, . . . , 4, 2), and each one pointing to a DRKjconsecutive bit veiled position in conversion array 6005, respectivesections 6006, 6007, . . . , 6008, . . . , 6009, and 6010 of 128 bitsize each. In other words, the password (or the client host ID) and theSRNG 6002 sequence of integers are uniquely associated.

Block 6005 introduces a conversion array which, without sacrificing anygenerality limitations of BitVU algorithm, has 128 equal sections 6006,6007, . . . , 6008, . . . , 6009, and 6010, with 128 bits per eachsection. FIG. 6 presents an exemplary case, when SRNG 6002 generated 128sequential integers 4, 127, . . . , 4, and 7. For this exemplary casedisclosed in FIG. 6, the 1^(st) bit of DRKj 6001 yh1 is put into the4^(th) bit position of first section 6006 instead of r1,4 bit; then the2^(nd) bit of DRKj 6001 yh2 6012 is put into 127^(th) bit position ofsecond section 6007 instead of r2,127 bit, and so on, until 127^(th) bitof DRKj 6001 is put into the 4^(th) position of 127^(th) section 6009instead of r127,4 bit. Ultimately, the 128^(th) bit of DRKj 6001 is putinto the 2^(nd) bit position of the 128^(th) section 6010 of conversionarray 6005 instead of r128,2 bit. Once all bites of DRKj are veiled inthis manner inside conversion array 6005, the entire conversion array6005 is encrypted with DRKj−1, and the message is sent to client 3002.

At client 3002, the encrypted conversion array is decrypted with DRKj−1,saved at client 3002, from the previous server message (step 3011 in theKEDIA algorithm, FIG. 3). Then the procedure, a reversed one as comparedto that which is described above for the BitVU algorithm on server 3001side, is applied. The user password (or the client host ID) saved at theclient platform in step 3007 of the KEDIA algorithm (see FIG. 3) issupplied in a hashed form as a seed to Client Sequential Random NumberGenerator (SRNG) 6003, identical to the one on the server 3001 side.This password (or host ID) triggers SRNG 6003 to generate the samesequence of integers as on server 3001 side before, that is 4, 127, . .. , 4, 2. Then, the logic placed on client 3002 used the first integer 4to extract DRKj 1^(st) byte yh1 from the 4^(th) position in 1^(st) 128bits section 6006 of conversion array 6005, and placed it back in the1^(st) position of DRKj 6001. Consequently, the second integer 127 isused to extract DRKj 2^(nd) bit from the 127^(th) position in 2^(nd) 128bits section 6007 of conversion array 6005, and place it back into the2^(nd) bit position of DRKj 6001. This procedure continues until,ultimately, the 127^(th) bit of DRKj yh127 is extracted from the 4^(th)bit position in 127^(th) 128 bits section 6009 of conversion array 6005,and placed back into 127^(th) bit position of DRKj 6001, as well as the128^(th) bit of DRKj yh128 6011 being extracted from the 2^(nd) bitposition in 128^(th) 128 byte size section 6010 of conversion array6005, and placed back into 128^(th) bit position of DRKj 6001. Thiscompletes the reassembling procedure of the BitVU algorithm to restoreDRKj at client 3002.

FIG. 7 is a block diagram of the Byte-Bit-Veil-Unveil (BBVU) algorithmaccording to the present invention. Block 7001 shows DRKj, where eachbyte is separated from a neighboring byte with a vertical bar. Withoutsacrificing any generality limitations of the BBVU algorithm, DRKj isassumed to be a 16-bytes key in FIG. 7. The user password (or the clienthost ID), supplied by server 3001 in a hashed form, plays a seed rolefor Server Sequential Random Number Generator (SRNG) 7002. SRNG 7002generates a random sequence of 16 integers, and then the server'sSequential Direct Bit Position Scrambler (SDBPS) 7006 scrambles all bitpositions in the veiled byte 7010. SDBPS 7006 generates a random seriesof non-repeating eight digits within the range from 1 to 8, for each ofSRNG 7002 integers in the sequence. In other words, the password (or theclient host ID), the SRNG 7002 sequence of integers, and the series ofdigits generated by SDBPS 7006 are uniquely associated. Applying thesame seed (the user password, or the server host ID, in a hashed form)will result in the same sequence of integers generated by SRNG 7002, andthe same series of digits generated by SDBPS 7007 for each integer inthe sequence.

Block 7006 introduces a conversion array which, without sacrificing anygenerality limitations of the BBVU algorithm, has 16 sections similar to7008, with 10 bytes per section. Similarly to the ByteVU algorithm, eachsection will veil one byte of DRKj 7001 in a position, respective to theparticular integer value generated by SRNG 7002. For instance, the1^(st) byte of DRKj 7001 xh1 occupies the 4^(th) byte position insection 7008, replacing r1,4 byte. FIG. 7 presents an exemplary case,where the 1^(st) DRKj byte xh1 has an 8-bit representation from the mostsignificant bit xh1,8 to the least significant bit xh1,1 7009, andchosen as 01011101 in FIG. 7 7009. SRNG 7002 generated 16 sequentialintegers 4, . . . , while SDBPS 7006 generated a series of eightnon-repeating digits 3, 1, 8, 5, 4, 2, 7, and 6 for the first integer 47011, and a similar series of digits for the rest of the integers.Eventually, all bits of the 1^(st) DRKj 7001 byte in 7008 occupy new bitpositions, consecutively specified in the SDBPS 7006 generated series ofdigits for the first integer 4. For a particular example in FIG. 7 7012,it is 01011011. The same process 7013 of scrambling bits for each veiledbyte of DRKj 7001 in conversion array 7007 is continued, until all bytesof DRKj are veiled, and all bit positions of each veiled byte arescrambled. Then, the entire conversion array 7007 is encrypted withDRKj−1, and the message is sent to client 3002.

At client 3002, the encrypted conversion array is decrypted with DRKj−1,saved at client 3002, from the previous server message (step 3011 inKEDIA, FIG. 3). Then the procedure, a reversed one as compared to thatwhich is described above for the BBVU algorithm on server 3001 side, isapplied. The user password (or the client host ID), saved at the clientplatform in step 3007 of the KEDIA algorithm (see FIG. 3), is suppliedin a hashed form as a seed to Client Sequential Random Number Generator(SRNG) 7005 identical to the one on the server 3001 side. This password(or host ID) triggers SRNG 7005 to generate the same sequence ofintegers as on server 3001 side before 4 Client Sequential Reverse BitPosition Scrambler (SRBPS) 7003 generates the reversed series of digitsfor each integer, as compared to its server counterpart SDBPS 7006. Forinstance, for the first integer 4, SRBPS 7003 generates the reversedseries 2, 6, 1, 5, 4, 8, 7, and 3, which allows the logic placed onclient side 3002 to restore bits in the original order for the 1^(st)byte of DRKj means that the 2^(nd) bit of the scrambled byte will becomethe least significant bit in the restored 1^(st) DRKj byte, and so on,until 3, the last digit in the series, is reached, indicating that the3^(rd) bit in the scrambled byte will become the most significant bit inthe restored 1^(st) byte. Meanwhile, integer 4 points to the 4^(th)position in section 7008 of conversion array 7007, where the 1^(st) DRKjbyte has been veiled. The same procedure continues, until all byes ofDRKj 7001 and their respective bits are returned to their originalpositions. This completes the reassembling procedure of the BBVUalgorithm to restore DRKj at client 3002.

At this time it is important to note that the ByteVU, BitVU, and BBVUalgorithms, disclosed above, require assessment of security of thesealgorithms against possible computer processing attacks now and in thefuture. Table 1 below presents a summary of this assessment. SRNG 5002,5003 (FIG. 5), 6002, 6003 (FIG. 6), and 7002, 7003 (FIG. 7) generateintegers pseudo-randomly, as well as SDBPS 7006 and SRBPS 7003 (FIG. 7).Hence, probabilities of veiling each byte and bit inside a ConversionArray (CA) for each algorithm can be viewed as independent ones. Bestmicroprocessors achieved ˜1 GHz clock rate barrier by the beginning ofthe 21^(st) century. Previously, forecasting allowed for at least 25-35years, until the clock rate would reach ˜(100-1000) GHz. Thus, currentlyavailable ˜1E10 instructions per second could reach ˜(1E12-1E13)instructions per second in a distant future, (assuming microprocessorRISC pipelined architecture with up to 10 stages per cycle). A veryconservative assumption is made that the attacking computers have 100%efficiency of their CPU utilization during an attack. In other words,testing each possible combination of all bytes, bits, or the combinationthereof, of a veiled message in CA will consume only one microprocessorinstruction.

Column 1001 in Table 1 presents particular geometries of CA chosen ineach algorithm for the assessment. Column 1002 gives the bit size ofeach algorithm CA for every geometry selected in 1001. Column 1003presents the total number of pseudo-random integers generated by SRNG ofeach algorithm with respect to the geometries chosen in 1001. Column1004 introduces probability models for each algorithm with respect tothe geometries of CA chosen in 1001. Every position in 1004 givesprobability to estimate the entire combination of veiled bytes, bits, orthe combination thereof, for each algorithm, under given geometry of CAin 1001. Column 1005 presents for each CA its transit time, given theslowest standard modem of 28.8 kbps (kilobits per second) ofcontemporary networks (for example, the Internet). Column 1006 presentsassessed time, required for a brute force attack now and in a distantfuture, for each algorithm and their respective geometries of CA chosenin 1001. Column 1007 presents an approximate time for one advancedmicroprocessor 1 GHz/100 GHz) instruction now, and in a distant future.

TABLE 1 1001 1002 1004 1005 1006 1007 ↓ ↓ 1003 ↓ ↓ ↓ ↓ CA Size CA ↓Prob- CA Transit Brute Force CPU One # of Total SRNG ability Time AttackInstruction rows vs. Bit total Model modem Time Time, (S) # of BB Size #for CA 28.8 kbps Now/Future Now/Future ByteVU 16r/16 bytes 2048 bits  16(1/16){circumflex over ( )}16 71 milliSec. 58 y/7 months 1E-10/1E-12BitVU 128r/2 bits  256 bits 128 (1/2){circumflex over ( )}128  9milliSec. 1E21 y/1E19 y 1E-10/1E-12 BBVU 16r/2 bytes  256 bits 144((.5)(1/8){circumflex over ( )}8){circumflex over ( )}16  9 milliSec.8E102y/8E100y 1E-10/1E-12

Summarizing the assessment results in Table 1, it can be noted that eachof ByteVU, BitVU, and BBVU algorithms give extremely high security nowand in a distant future for the respective geometries selected in 1001.At the same time, one CA message transit times 1005, even for theslowest standard modems, are reasonable enough for the disclosedalgorithms' practical utilization in the MEDIA protocol. Certainly,geometry parameters in 1001 can be regarded as security parameters ofthe MEDIA protocol, and these parameter changes could allow for securitytrade-offs vs. cost (CPU power of client/server or authenticator/peerplatforms, and the network throughput). Also, replacing slow modems bycontemporary high-speed network connections, like DSL, wouldsignificantly reduce message transit times in 1005.

The combination of the KEDIA algorithm and any one of ByteVU, BitVU, andBBVU algorithms comprise the fourth security tier, which makes theencrypted authentication protocol highly secure against online andoffline attacks. The algorithms described above allow for the encryptionkey management security to be scaled with CPU and network throughputresources. During the encryption key distribution stage overcommunication lines, shared secrets never leave the server, or theclient. However, they are repeatedly employed for each iterative messageencryption/decryption by KEDIA and any of ByteVU, BitVU, or BBVUalgorithms on the server and the client platform as well. Only when theclient and the server eventually have in their possession the FinalSecret Key (FSK) satisfying the required security level, then the serverand the client will perform mutual authentication in a way that neitherof authentication credentials enter communication lines in either form.The authentication session is denied, provided the parties' mutualauthentication is not successfully completed. This part of the encryptedauthentication protocol completes the client/server mutualauthentication. At the same time, it is the final fifth security tier ofthe encrypted authentication protocol.

FIG. 8A and FIG. 8B illustrate the server and the client side of theMessage Encrypt/Decrypt Iterative Authentication (MEDIA) protocolaccording to the present invention. Without sacrificing any generalitylimitations of the MEDIA protocol, the exemplary case presented in FIG.8A and FIG. 8B is assuming HTTP communication protocol (RFC 2068Hypertext Transfer Protocol—HTTP/1.1 January 1997), Java applet/servletmulti-threading object-oriented communication technology, and a standardWeb server technology. However, the MEDIA protocol can be integratedinto any other network communication protocol, and enabled with variousobject-oriented technologies. The ByteVU algorithm has been includedinto the MEDIA protocol in FIG. 8A and FIG. 8B, though any of BitVU andBBVU algorithms could serve there equally well.

Messages sent to the client and received at the server are numbered in8000. Key functional message destinations on the server side are in8001, and on the client side they are in 8016. For each message receivedat the server, its content description is in 8003, whereas for eachmessage received at the client, its content description is in 8014.Similarly, for each message sent from the server, its contentdescription is in 8002, whereas for each message sent from the client,its content description is in 8015. The choice of any one of ByteVU,BitVU, or BBVU algorithms to be used in the MEDIA protocol and theparameters of the respective conversion array are in 8006 for the serverside, and in 8010 for the client side. Seeds, having been used totrigger SRNG (Sequential Random Number Generator), are in 8007 for theserver side, and they are in 8009 for the client side. Which direction aparticular MEDIA message is sent towards, is in 8008. The ByteVUalgorithm conversion array parameters, chosen in FIG. 8A and FIG. 8B (10sections with 25 bytes size of each), give extremely high securityprotection against online and offline intruding attacks, even for oneMEDIA message as it was shown above. Therefore, it is practicallyjustifiable to reduce iterations in the KEDIA algorithm by limiting DRKnin FIG. 3 to DRK2 only. It saves client/server platforms CPU and networkresources, while keeping a very high security level.

It is assumed, without sacrificing any generality of the MEDIA protocol,that for this particular embodiment of the MEDIA protocol (FIG. 8A andFIG. 8B), the client is a user at the client platform. The communicationsession begins with the user's request (message 1) to the server toreach a protected network resource, for example, a URL (UniversalResource Locator), a protected link, a protected file, a protecteddirectory, or another protected network resource. This message initiatesthe MEDIA protocol on the server side. The server replies to the user(message 2), sending SRK 1011 (see FIG. 1) over the communication line(the Internet) in a compiled class form, which prevents any easy keyreuse or reengineering, if it is intercepted by an intruder. The userenters into the GUI (Graphical User Interface, designed into the appletand sent from the server to the client in message 2 along with the SRK)the user name, the user password, and the server password. The passwordsstay stored at the client, while the user name gets encrypted with theSRK and sent to the server in message 3.

The server (logic on the server side in this exemplary case could beimplemented in the Java servlet technology) replies in message 4 withDRK1 2012 (FIG. 2) bytes veiled with the ByteVU algorithm, triggered bythe server, supplying the hashed password of the assumed user as a seed.The resulting ByteVU conversion array is encrypted with the SRK and sentto the client. The client, having known the SRK and the user password,entered into the GUI in the previous message 3, decrypts the conversionarray and reassembles DRK1 bytes. In message 5, from the client to theserver, hashed DRK1 bytes are veiled with ByteVU algorithm, triggered bythe user password, stored at the client earlier in step 3 (FIG. 8B), andconverted to its hash equivalent. Then, the conversion array isencrypted with DRK1 and sent to the server, which decrypts theconversion array with DRK1, and triggers ByteVU with the hashed passwordof the assumed user, taken from the database attached to the server. Ifthe hashed DRK1 is correct, reassembled in this way, it is actually theauthentication signal from the client to the server, as nobody exceptthe client knows the user password used to trigger the ByteVU algorithmwhen receiving message 4, and sending message 5.

If DRK1 is incorrect, the MEDIA protocol is terminated by the serversending a “no” authentication message (or an error message: “userpassword is incorrect”) to the client, encrypted with SRK. Otherwise,the server sends to the client message 6 containing DRK2, which bytesare disassembled by the ByteVU algorithm, triggered by the user hashedpassword, used as a seed for SRNG 5002 (FIG. 5). Then, the conversionarray is encrypted with DRK1 and sent to the client, where it isdecrypted with DRK1 stored at the client from the previous message 5,and DRK2 bytes get reassembled by the ByteVU algorithm, triggered by theuser password, stored at the client earlier in step 3 (FIG. 8B). Theclient replies to the server with message 7, sending to the serverhashed DRK2, which bytes are veiled by the ByteVU algorithm, triggeredby the user password, stored at the client in the previous message 3,and converted to its hash equivalent. The server decrypts message 7 fromthe client with DRK2, and reassembles the hashed DRK2 bytes with theByteVU algorithm, triggered by the user password, taken from theattached to the server database, and converted to its hash equivalent.If DRK2 is correct, the server sends to the client message 8 with DRK2,which bytes are disassembled with the ByteVU algorithm, triggered by theserver password. Otherwise, if DRK2 is not correct, the MEDIA protocolis terminated. The conversion array of the ByteVU algorithm in message 8is encrypted with DRK2 and sent to the client.

The client, receiving message 8 from the server, decrypts it with DRK2,and reassembles the hashed DRK2 bytes with the ByteVU algorithm,triggered by the server password, stored on the client side in message3. Then, the client compares the decrypted and reassembled DRK2 withDRK2 from the previous message 6. If they are the same, it is viewed bythe client as the authentication signal from the server, because onlythe client and server share the server password. Hence, it was the onlyserver, which could send the last message 8 to the client. Now, as thetrust is established by the client to the server, the client sends tothe server message 9 with hashed DRK2, which bytes are disassembled withthe ByteVU algorithm, triggered by the server password, stored on theclient side in message 3, and converted to its hash equivalent.Eventually, the conversion array of the ByteVU algorithm is encryptedwith DRK2 and sent to the server. The server, having received message 9from the client, decrypts it with DRK2, and reassembles the hashed DRK2bytes with the ByteVU algorithm, triggered by the hashed serverpassword. If DRK2 is correct, it is viewed by the server as a secondauthentication factor from the client (the client confirmed the serverpassword), in addition to the first factor, having been checked in themessage 6 from the client (the client confirmed the user password).

This completes the mutual authentication of the client/server pairaccording to the MEDIA protocol, and the server is now ready to make anauthentication decision. In the end, the server sends to the clientmessage 10, which has either a “go” authentication signal, assuming DRK2in message 9 from the client was correct, or an error message: “theserver password is incorrect”, assuming DRK2 in message 9 from theclient was incorrect. Each signal byte is disassembled with the ByteVUalgorithm, triggered by the user password from the database, attached tothe server, and then the conversion array of the ByteVU algorithm isencrypted with DRK1 and sent to the client in message 10. Havingreceived the message 10, the client decrypts it with DRK1, stored at theclient platform during message 4, and reassembles the signal bytes withthe ByteVU algorithm, triggered by the user password, stored at theclient side in message 3.

This effectively completes the entire MEDIA protocol of theclient/server communication session as presented in FIG. 8A and FIG. 8B.As one can see, authentication credentials (the user password and theserver password in this particular embodiment) have never passed throughcommunication lines in any form. Also, the client/server mutualauthentication has been completed within the MEDIA protocol, as well asthe exchange of FSK (Final Secret Key, which is DRK2 in this particularembodiment) having been performed within the client/server pair. Theserver password and the user password enable secure mutualauthentication, according to the MEDIA protocol architecture. At thesame time, they are both playing a role of a strong two-factorauthentication of the client at the server platform.

FIG. 9 illustrates the Graphical User Interface (GUI) enablingclient/server mutual authentication at the client platform according tothe MEDIA protocol, and a graphical illustration of the distributedprotected network resources, including the authentication server, andthe user base the MEDIA protocol is used for, according to the presentinvention. This GUI has already been mentioned or assumed along with thepreferred embodiments of this invention, described herein, for instance,in FIG. 3 step 3 3007, FIG. 8B messages 3, 5A, and 10 8016. The user ona client platform 9015, or 9021 in FIG. 9 is trying to reach a protectednetwork destination 9020. It invokes the MEDIA protocol through aninteractive communication session between web server 9018, computeserver 9024, program logic 9017, and security and account databases 9022and 9023, all located on the server side, with GUI 9003 located on theclient side. There are different means to implement this scheme, forexample, thick or thin software client, permanently placed on a clientplatform, or a Java applet, loading GUI 9003, and its respectiveclient-side logic into a browser. The latter case in the preferredembodiment in FIG. 9 is assumed here. Also, the network, over which thecommunication session is established, could be either only LAN (LocalArea Network), or WAN (Wide Area Network), or a combination of LAN andWAN together. In the particular embodiment in FIG. 9, Internet 9019 isassumed as a preferred embodiment, enabling client/server dialoguethrough communication links 9016.

GUI 9003 has several operation modes 9009: login session mode 9010,account set-up mode 9011, user password reset mode 9012, and serverpassword reset mode 9013. Login session 9010 is the default operationmode. The user enters the user name in window 9004, the user password inwindow 9005, and the server password in window 9006. The user has achoice to enter alphanumeric characters, or their echo dots for securityreasons by toggling button 9014. The session elapsed time clock 9007visualizes this value to the user, and signals communication sessiontermination once the session time has expired. After the authenticationcredentials are all entered into 9004, 9005, and 9006, the clientindicates login button 9008, which completes step 3 3007 in FIG. 3, ormessage 3 in FIG. 8B. Then the other steps of the MEDIA protocol areinitiated. Stoplight 9001 turns yellow, when button 9008 is indicated,signaling the MEDIA protocol is in progress for the first authenticationfactor (the user password) examination. Message 8 in FIG. 8B, havingarrived at the client, initiates stoplight 9001 to change color from redat the beginning of the session to green, once it is checked by theclient placed logic that DRK2 delivered in the message 8 is identical toDRK2, delivered in message 6.

Similarly, stoplight 9002 turns from red to the yellow color right afterstoplight 9001 turned green, signaling that the MEDIA protocol is inprogress for the second authentication factor (the server password)examination. Indeed, once the client received message 10 in FIG. 8B,stoplight 9002 turns green, signaling successful client/server mutualauthentication, FSK exchange, and completion of the MEDIA protocol. Ifthe client received message 5A from the server (FIG. 8A and FIG. 8B),stoplight 9001 turns red, back from the yellow color, and the errormessage “the user password is incorrect” appears in system window 9014,signaling the MEDIA protocol termination. Also, if the client receivedauthentication signal “no” in message 10 from the server (FIG. 8A andFIG. 8B), stoplight 9002 turns red, back from the yellow color, and theerror message “the server password is incorrect” appears in systemwindow 9014, signaling the MEDIA protocol termination.

Though, a server password unique to each user remains the preferredembodiment of this invention, various business environments, orenterprise/organization/agency IT resource configurations may requiresome modifications to the MEDIA protocol. The exemplary case would bewhen users of all computer platforms logged-in from the same server inan isolated LAN environment (or the same cluster of servers). Then thesystem administrator may preset the same server password at allplatforms, during each platform configuration and setup on the network.This would require any user to enter only the user name, and the userpassword in GUI 9003 inside an enterprise, organization, or agency.Alternatively, messages 8 and 9 in the MEDIA protocol (FIG. 9) could beeliminated entirely for the above case, which effectively excludes theneed for server password to perform a user (a client platform)authentication and a session key exchange. However, any connection withservers and users outside the particular LAN perimeter would probablyrequire the reinstatement of server passwords for security reasons.

While the present invention is disclosed by reference to the preferredembodiments and examples detailed above, it is to be understood thatthese examples are intended in an illustrative rather than in a limitingsense. It is contemplated that modifications and combinations willreadily occur to those skilled in the art, which modifications andcombinations will be within the spirit of the invention and the scope ofthe following claims.

1. A method for mutual authentication in communications between firstand second stations, comprising: generating and storing a set ofephemeral session keys at the first station, ephemeral session keys inthe set being associated with respective session key initiationintervals, and being discarded upon expiry of respective session keylifetimes at a time later than expiration of the respective session keyinitiation intervals; in response to a request to initiate acommunication session received by the first station during a particularsession key initiation interval, selecting the associated session key;sending a message carrying said associated session key to the secondstation, and receiving a response from the second station including adigital identifier, the digital identifier being information sharedbetween the first station and the second station, or between the firststation and a user at the second station, the digital identifier beingencrypted using said associated session key to verify receipt of thesession key by the second station and to identify the second station orthe user of the second station; generating and storing, in the firststation, a set of intermediate data keys, the set of intermediate datakeys including intermediate data key (i), for i=1 to at least n, where nis at least 2, and being discarded at a time later than expiration ofthe particular session key initiation interval; executing a first set ofexchanges including one or more exchanges with the second station, afterverifying in said first station receipt of the session key by the secondstation by decrypting the digital identifier using the associatedsession key at the first station and positively matching the decrypteddigital identifier against an existing entry in a stored list ofauthorized users, the first set of exchanges including in the i^(th)exchange sending a message to the second station carrying intermediatedata key (i) from said set of intermediate data keys encrypted using theassociated session key for a first exchange in first set of exchangesand using the intermediate data key (i−1) for subsequent exchanges inthe first set of exchanges, receiving a response from the second stationincluding a hashed version of intermediate data key (i) encrypted usingintermediate data key (i), decrypting the hashed version of theintermediate data key (i), calculating a hashed version of intermediatedata key (i) at the first station, and matching the calculated hashedversion and the received hashed version of intermediate data key (i) toverify receipt by the second station of intermediate data key (i);executing a second set of exchanges for mutual authentication afterverifying in said first station receipt of the intermediate data key(n−1) by the second station, including sending a first message carryingintermediate data key (n) encrypted using a hashed version of a firstshared secret, receiving a response from the second station carrying ahashed version of intermediate data key (n) encrypted using a hashedversion of the first shared secret, and decrypting the hashed version ofthe intermediate data key (n), calculating a hashed version ofintermediate data key (n) at the first station, and matching thecalculated hashed version and the decrypted hashed version ofintermediate data key (n) to verify possession by the second station ofthe first shared secret; sending a second message carrying intermediatedata key (n) encrypted using a hashed version of a second shared secret;and if the second station sends a response to the second message,carrying a hashed version of intermediate data key (n) encrypted using ahashed version of the second shared secret, after possession by thefirst station of the second shared secret is verified at the secondstation, the verifying being accomplished at the second station bydecrypting the intermediate data key (n) from the second message usingthe hashed version of the second shared secret, calculating a hashedversion of the intermediate data key (n), and matching the calculatedhashed version and the decrypted hashed version of intermediate data key(n) to verify possession by the first station of the second sharedsecret, then receiving the response from the second station, anddecrypting the hashed version of the intermediate data key (n) using thehashed version of the second shared secret, calculating a hashed versionof intermediate data key (n) at the first station, and matching thecalculated hashed version and the decrypted hashed version ofintermediate data key (n) at the first station to verify mutualauthentication of the first and second stations; and if mutualauthentication is verified at the first station, then sending a messageindicating successful authentication; wherein the session key lifetimeshave respective lengths which are longer than said session keyinitiation intervals, and less than a multiple M times a time requiredfor verification of mutual authentication using said first and secondsets of exchanges in expected circumstances, where M is less than orequal to
 10. 2. The method of claim 1, including using said associatedsession key in response to another request to initiate a communicationsession from a third station received by the first station during saidparticular session key initiation interval, and using other session keysfrom the set of ephemeral session keys after expiry of said particularsession key initiation interval.
 3. The method of claim 2, includingassociating a unique set of intermediate data keys with each sessionkey.
 4. The method of claim 1, including: providing a buffer at thefirst station; storing the set of ephemeral session keys in the buffer;and removing session keys from said buffer upon expiry of respectivesession key lifetime.
 5. The method of claim 4, wherein the session keylifetimes have respective lengths longer or equal to a time required forverification of mutual authentication using said first and second setsof exchanges in expected circumstances.
 6. The method of claim 1,wherein said message indicating successful authentication carries asignal encrypted using intermediate data key (n−1) or using anotherprearranged one of said intermediate data keys (i).
 7. The method ofclaim 1, including using intermediate data key (n) as a symmetrical keyto encrypt data during post-authentication in communications between thefirst and second stations in the communication session.
 8. A dataprocessing apparatus, comprising: a processor associated with a firststation, a communication interface adapted for connection to acommunication medium, and memory storing instructions for execution bythe data processor, the instructions including logic to receive arequest via the communication interface for initiation of acommunication session between a first station and a second station;logic to provide for mutual authentication in communications between thefirst station and a second station, comprising: generating and storing aset of ephemeral session keys at the first station, ephemeral sessionkeys in the set being associated with respective session key initiationintervals, and being discarded upon expiry of respective session keylifetimes at a time later than expiration of the respective session keyinitiation intervals; in response to a request to initiate acommunication session received by the first station during a particularsession key initiation interval, selecting the associated session key;sending a message carrying said associated session key to the secondstation, and receiving a response from the second station including adigital identifier, the digital identifier being information sharedbetween the first station and the second station, or between the firststation and a user at the second station, the digital identifier beingencrypted using said associated session key to verify receipt of thesession key by the second station and to identify the second station orthe user of the second station; generating and storing, in the firststation, a set of intermediate data keys, the set of intermediate datakeys including intermediate data key (i), for i=1 to at least n, where nis at least 2, and being discarded at a time later than expiration ofthe particular session key initiation interval; executing a first set ofexchanges including one or more exchanges with the second station, afterverifying in said first station receipt of the session key by the secondstation by decrypting the digital identifier using the associatedsession key at the first station and positively matching the decrypteddigital identifier against an existing entry in a stored list ofauthorized users, the first set of exchanges including in the i^(th)exchange sending a message to the second station carrying intermediatedata key (i) from said set of intermediate data keys encrypted using theassociated session key for a first exchange in first set of exchangesand using the intermediate data key (i−1) for subsequent exchanges inthe first set of exchanges, receiving a response from the second stationincluding a hashed version of intermediate data key (i) encrypted usingintermediate data key (i), and decrypting the hashed version of theintermediate data key (i), calculating a hashed version of intermediatedata key (i) at the first station, and matching the calculated hashedversion and the received hashed version of intermediate data key (i) toverify receipt by the second station of intermediate data key (i);executing a second set of exchanges for mutual authentication afterverifying in said first station receipt of the intermediate data key(n−1) by the second station, including sending a first message carryingintermediate data key (n) encrypted using a hashed version of a firstshared secret, receiving a response from the second station carrying ahashed version of intermediate data key (n) encrypted using a hashedversion of the first shared secret, and decrypting the hashed version ofthe intermediate data key (n), calculating a hashed version ofintermediate data key (n) at the first station, and matching thecalculated hashed version and the decrypted hashed version ofintermediate data key (n) to verify possession by the second station ofthe first shared secret; sending a second message carrying intermediatedata key (n) encrypted using a hashed version of a second shared secret;and if the second station sends a response to the second message,carrying a hashed version of intermediate data key (n) encrypted using ahashed version of the second shared secret, after possession by thefirst station of the second shared secret is verified at the secondstation, the verifying being accomplished at the second station bydecrypting the intermediate data key (n) from the second message usingthe hashed version of the second shared secret, calculating a hashedversion of the intermediate data key (n), and matching the calculatedhashed version and the decrypted hashed version of intermediate data key(n) to verify possession by the first station of the second sharedsecret, then receiving the response from the second station, anddecrypting the hashed version of the intermediate data key (n) using thehashed version of the second shared secret, calculating a hashed versionof intermediate data key (n) at the first station, and matching thecalculated hashed version and the decrypted hashed version ofintermediate data key (n) at the first station to verify mutualauthentication of the first and second stations; and if mutualauthentication is verified at the first station, then sending a messageindicating successful authentication: wherein the session key lifetimeshave respective lengths which are longer than said session keyinitiation intervals, and less than a multiple M times a time requiredfor verification of mutual authentication using said first and secondsets of exchanges in expected circumstances, where M is less than orequal to
 10. 9. The apparatus of claim 8, including logic to use saidassociated session key in response to another request to initiate acommunication session from a third station received by the first stationduring said particular session key initiation interval, and using othersession keys from the set of ephemeral session keys after expiry of saidparticular session key initiation interval.
 10. The apparatus of claim9, including logic to associate a unique set of intermediate data keyswith each session key.
 11. The apparatus of claim 8, including a bufferat the first station; logic to store the set of ephemeral session keysin the buffer and to remove session keys in said set of ephemeralsession keys from said buffer after expiry of the respective session keylifetimes.
 12. The apparatus of claim 11, wherein the session keylifetimes have respective lengths longer or equal to a time required forverification of mutual authentication using said first and second setsof exchanges.
 13. The apparatus of claim 8, wherein said messageindicating successful authentication carries a signal encrypted usingintermediate data key (n−1) or using another prearranged one of saidintermediate data keys (i).
 14. The apparatus of claim 8, includingusing intermediate data key (n) as a symmetrical key to encrypt dataduring post-authentication communications between the first and secondstations in the communication session.